From Brian Brokaw:

Recently Ron Foster (who translates the Pocket Monster cards for
Wizards of the Coast--the "official" translations and the _REAL_
creator of "Haymaker") emailed me regarding a sticky rule situation.

Does Ditto have a memory?

Specifically, if Ditto is Poisoned, Smokescreened, or otherwise
"effected", will this "effect" go away if Ditto changes form?
(because the opponent changed Pokemon and Ditto Transformed to match)

Typically, whenever a Pokemon is removed from the active position
or evolved, ALL "effects" that may be lingering with that Pokemon 
are removed.  When Ditto's opponent evolves, it will also appear
as if Ditto has "evolved" because it will immediately begin copying
the new defending evolution card. So, does Ditto get to cure itself
of lingering "effects" when this happens or not?

The answer, direct from Japan, is NO.
Ditto has a memory and will retain all "effects" even after
Transforming to match a new/different Pokemon.

Below is Ron's response detailing this answer:

From: Foster, Ronald
Sent: Wednesday, December 01, 1999 11:02 PM
To: 'brian brokaw'
Subject: RE: poisoning metamon

[[ included Robert in the BCC ]]

Brian, 

I received an answer from Media Factory to your question about a Poisoned
Metamon, er, Ditto. (For Robert's benefit, I'll include the Romanized
Japanese answer at the end of the letter.)

Their answer is: If Ditto is affected by Poison or "Smokescreen" or a
similar effect, that effect will not wear off unless Ditto stops being the
Active Pokemon. This is because even if Ditto copies another Pokemon, the
physical card "Ditto" is still in play.

To compare this to Magic, a Mishra's Factory or Treetop Village that turns
into a creature is not treated as a "creature coming into play"--the actual
_card_ itself was always in play. Same with Ditto--even if he's copying
another Pokemon, he's still Ditto (after all, he retains his copy ability
and his "I can use any Energy" ability), so if he's Poisoned while copying a
Blastoise, he stays Poisoned if he turns into a Charizard. Ditto's not
Evolving, Ditto is just changing form. By extrapolation, I imagine this
ruling affects Blaine's Ninetails as well.